I totally agree with Transsonic2000, i personnaly used BSG, aviationexam and easyATPL and they are all pretty much similar in term of questions.
However you should probably pick the one that offer the format that you prefere. For example with the BSG platform, you can go to the next question by pressing the arrow (do not seem much but when you do thousand of questions it is really nice to not use your mouse to click on the next question). Aviationexam offers Apps for IOs and Android... Really nice as well. I really like the summary/explantation that they provide as well.
Just try both for a couple days (free trials)
